Oxidized glutathione has the same actions as reduced glutathione, and as actions of oxidized glutathione, for example, hepatic detoxification action via oral administration, and the like are known (Non-Patent Document 1). Due to this, oxidized glutathione may be used for various applications in which reduced glutathione has been used, and is useful, for example, as a product, raw material or intermediate of health-aid food, pharmaceuticals, cosmetics, and the like.
Known process for producing oxidized glutathione include a process in which an aqueous solution or yeast extract solution containing oxidized glutathione obtained by a fermentation process, enzymatic process or the like is used as a starting material, and in the aqueous solution or yeast extract solution, the reduced glutathione is converted into the oxidized glutathione to obtain a solution of oxidized glutathione, and then by conducting a concentration, a dilution and the like, an aqueous solution or yeast extract solution containing the oxidized glutathione is obtained (Patent Documents 1 and 2); a method for obtaining powder of yeast extract solution containing oxidized glutathione by adding excipients and the like, and freeze-drying, spray-drying or the like (Patent Document 2); a process for producing a crystal of oxidized glutathione monohydrate by adding alcohol and the like dropwise to an aqueous solution containing oxidized glutathione (Patent Document 3); a process for producing a crystal of oxidized glutathione octahydrate (Non-Patent document 2) and the like.
However, it is known that the production method by freeze-drying a solution of oxidized glutathione shows poor impurity selectivity and require a large amount of energy for freeze-drying so that it is not suitable for industrialization. Meanwhile, the method based on spray-drying is known to show an increase in impurities due to thermal contact.
In addition, it is known that the crystal of oxidized glutathione octahydrate is problematic in that the content of water contained in the crystal is not uniform, and shows poor stability and a long time up to 3-4 days is required to obtain the crystal, and that the crystal of oxidized glutathione monohydrate leave room for improvement in that crystal separation capability is poor because it is a needle-like crystal and easily agglomerate, impurity selectivity (purification ability) is poor and crystal growth rate is slow.
Therefore, there is a need for a crystal of oxidized glutathione that has excellent stability and is handled industrially with ease.